1. Field of the Invention
The present invention relates to a multi-beam scanning method, apparatus and multi-beam light source device using a plurality of semiconductor laser array units each having a plurality of light emitting points.
2. Discussion of the Background
It is known to use a semiconductor laser array unit as a light source in a multi-beam scanning apparatus in which a plurality scanning lines are simultaneously scanned on a scanning surface with a plurality of beams which are formed as a plurality of beam spots. The beam spots are separated from each other in a sub-scanning direction on the scanning surface. The semiconductor laser array unit includes a monolithic semiconductor laser array chip having a plurality of light emitting points (laser light emitting elements) which are arranged in a row.
Japanese Patent Laid-open Publication No. 3-107910 proposes to use a semiconductor laser array unit in which two light emitting points are arranged 100 μm apart from each other.
When the imaging magnification of an optical system arranged in a light path extending from the light source to a scanning surface is considered, the distance of 100 μm is too large because when the two light emitting points are arranged in the sub scanning direction, the pitch of the scanning lines (the distance between two adjacent scanning lines) becomes excessively large.
Therefore, in JP 3-107910, a desired smaller scanning line pitch is accomplished by slightly inclining the semiconductor laser array unit such that the direction in which the light emitting points are arranged in the semiconductor laser array unit is inclined relative to the main scanning direction.
Generally, in optical scanning apparatuses, to achieve surface tilt correction for an optical deflector, the image forming magnification of an optical system which is arranged between a light source and a scanning surface differs between the main and sub scanning directions. As a result, the angle of a direction in which light emitting points are arranged in a semiconductor laser array unit relative to the main scanning direction differs from the angle of a direction in which beam spots are arranged on the scanning surface relative to the main scanning direction.
Because of this difference, it is very difficult to accurately adjust the incline angle of the semiconductor laser array unit so that a desired scanning line pitch is accomplished. Therefore, it is desirable that the semiconductor laser array unit can be used so that the arrangement direction of the light emitting points is the same as the sub scanning direction.
If the distance between the light emitting points is reduced to be very small, it is possible to prevent the scanning light pitch from becoming excessively large even when the light emitting points are arranged in a direction in accordance with the sub scanning direction. For example, if a semiconductor laser array unit in which light emitting points are spaced apart from each other by about 5 μm-7 μm is used, a sufficiently fine scanning line pitch is achieved even though the arrangement direction of the light emitting points is in accordance with the sub scanning direction. However, the manufacturing method required to produce such semiconductor laser array unit having a finer and smaller interval between light emitting points is complicated and therefor, the manufacturing cost is relatively high. As a result, the cost of a multi-beam scanning apparatus using such a semiconductor laser array unit is very high.
The manufacture of a semiconductor laser array unit becomes relatively easy if the distance between light emitting points is about 10 μm, and therefore the manufacturing cost of the semiconductor laser array unit becomes relatively low. However, if such semiconductor laser array unit having the light emitting point interval of 10 μm is used with the arrangement direction of the light emitting points is in accordance with the sub scanning direction, the scanning line pitch on the scanning surface becomes excessively large, particularly when a fine density scanning line pitch is desired.